Many types of resins have heretofore been added to a coal-tar pitch to take advantage of its low cost and durability and to overcome its mechanical weakness, brittleness and poor adhesion. Thus, Downey U.S. Pat. No. 3,219,064 provides a pipe coating tape comprising coal-tar pitch, polyvinyl chloride and a plasticizer and Parkinson U.S. Pat. No. 3,361,692 provides a coating and adhesive composition comprising coal-tar pitch and an ethylene-vinyl acetate copolymer. Although these as well as other prior art compositions do enhance the suitability of coal-tar pitch for use as an adhesive and coating, the resulting compositions are either too expensive or do not have sufficiently good adhesive properties to provide a generally useful composition.
Olefinic polymers are known to be inert and flexible but exhibit poor adherence to most substrates and have poor stress crack resistance. Thus, Baum, U.S. Pat. No. 3,410,928, to provide an olefinic polymer composition having increased adhesive and stress resistant properties, adds an ethylene-acrylic co- or terpolymer to his olefinic polymer to provide a printable, grease resistant composition with improved adhesion. Such compositions however have poor abrasion and heat resistance, and are therefore not suitable for use in applications where such qualities are desirable.
Coal-tar pitch and asphalt compositions have been used to coat galvanized steel culverts to improve abrasion and corrosion resistance. Asphalt dipped culvert coating is a thick (50 mils) coating obtained by dipping sections of formed culvert pipe into molten asphalt. Disadvantages associated with this coating are (1) a non-uniform coating thickness, (2) poor adhesion, (3) poor low temperature performance, (4) the coating makes the pipe sticky and difficult to handle, (5) fire hazard, (6) the size of the dip tank puts a limit on the length of culvert sections possible, (7) subject to microbial attack, and (8) hydrogen sulfide permeability, and (9) air pollution hazard.
Asbestos bonded culvert coatings are also used. Sheets of asbestos, impregnated with asphalt are bonded to steel directly following the galvanizing operation using the still molten zinc as the bonding agent. Disadvantages associated with this product are (1) its high cost, (2) dependence upon sheet galvanizing operation, (3) availability only in cut lengths not suitable for spiral wound culverts, (4) subject to microbial attack, (5) hydrogen sulfide permeable, (6) poor control of zinc coating thickness and (7) difficulties in handling and shipping, (8) fire hazard in dip tank, and (9) air pollution hazard.
Ideally, a culvert coating should possess the following characteristics: (1) excellent abrasion and corrosion resistance, (2) excellent adhesion and uniformly of coating thickness, (3) low cost, (4) suitable for coating coils for use in spiral culvert machines, (5) elimination of the dip coating step, (6) clean and easy to handle and ship, (7) hydrogen sulfide resistant, and (8) resistant to microbial attack.